The invention relates to a process for the transport of metal ions from an aqueous liquid A (herein liquid A) to a nonaqueous liquid B (herein liquid B) and from the nonaqueous liquid B to an aqueous liquid Z (herein liquid Z) wherein, in a first transport zone, B receives the metal ions from A and is then passed to a second transport zone where it transfers the metal ions to Z. Furthermore, the liquid B is substantially immiscible with the liquids A and Z and contains at least one complexing agent for the metal ions and constitutes a solvent for the complexed metal ions.
Processes in which metal ions are to be removed from one aqueous solution to another aqueous solution are, for example, used in ore dressing. In this case, it is frequently desirable to selectively transfer ions of a certain type from an aqueous liquid and to obtain the pure metal therefrom. Another field of application for these processes is wastewater treatment, where metal ions are to be removed from aqueous liquids such that only a few traces of the metal ions remain.
Processes are known involving the formation of a complex for the transport of metal ions from one aqueous solution to another. West German Laid-open Application No. 29,10,793, for example, describes a process in which an organic liquid containing a complexing agent is present in the pores of a membrane. Each surface of the membrane is in contact with the aqueous liquid transferring metal ions and with that accepting the same. Thus, the transport of metal ions is carried out by complexation at a membrane surface, diffusion of the complexes through the pore system, and release of the ions at the other membrane surface. The disadvantage of this type of process is that the complexing agent can escape from the pore system and enter the aqueous liquids. This can be caused by a certain solubility of the complexing agent in water or by pressure variations or turbulences in the aqueous liquids on the membrane surfaces. Initially, these losses of complexing agents lead to the slowing down of the process, i.e., to a decrease in flow of metal ions and, finally, to a suspension of the process. Before reuse, the membranes must be cleaned and refilled.
The problem described above is obviated in other processes by not using a complexing agent that is fixed in membrane pores. Rather, the organic liquid containing the complexing agent is passed from a first to a second transport zone. In the first transport zone, the metal ions are received by the complexing agent from an aqueous liquid and in the second transport zone they are transferred to another aqueous liquid. Such processes, operating according to the principles of extraction and separation of liquid systems by isolation, have been described in Chemical Engineering 1970, p. 82ff and in U.S. Pat. No. 3,939,203. These processes, too, have disadvantages. Undesirable admixtures can enter the organic liquid from the metal-ion-transferring aqueous liquid during the time when the two are thoroughly mixed. These admixtures must then be removed before recovery of the pure metal by purification operations. Another disadvantage is that during the mixing of the aqueous with the organic liquid, the metal ions, following the distribution law or the chemical equilibrium law, are distributed among the two liquids so that several mixing and separation steps are necessary to obtain high metal yields. As a rule, if these processes are applied to wastewater treatment, another disadvantage is recognized. Since wastewaters frequently contain surfactants, emulsification of the organic liquid or a portion thereof may occur, thus making it more difficult to isolate it from the aqueous phase.
As a consequence, processes in which both the metal-ion-transferring and the metal-ion-receiving aqueous liquids are each combined with the organic phase, then separated again therefrom in the usual manner, e.g., by isolation, often require a series of repetitive steps and are therefore costly.